Zirconium components are especially preferred in nuclear reactor systems such as nuclear fuel cladding. As described in my co-pending application Ser. No. 888,293 filed Jul. 22, 1986 and now Pat. No. 4,738,747, assigned to the assignee of the present invention, and incorporated by reference herein, zirconium alloy tubes are pilgered to reduce the size thereof, and are subsequently etched to remove defects from the tubing surface. The preferred zirconium alloys for use in nuclear fuel cladding include Zircaloy-2 and Zircaloy-4. An aqueous hydrofluoric acid - nitric acid etching bath is the preferred etching medium. It is known that the etching rate of such an aqueous bath decreases with use, upon dissolution of zirconium into the bath, until a limiting rate of about 20 percent of the fresh or initial bath is reached. At such a stage, the used or spent bath, which will generally contain about 24 g/l of dissolved zirconium alloy was discarded. The spent bath was treated to render it disposable before being discarded. The spent bath contains, among other components, various zirconium compounds or complexes, some tin components when Zircaloy is etched, and residual hydrofluoric and nitric acids.
In my previous co-pending application, Ser. No. 888,293, I provided a process for etching zirconium articles where a spent bath is replenished in acid to increase the etching rate thereof without the need to remove dissolved zirconium from the bath. Such a process extends the life of a hydrofluoric acid-nitric acid etching bath without a need for precipitating and removing dissolved zirconium material therefrom.
In order for an operator to know when an etching bath is spent to the extent that the same should be either regenerated or discarded, the dissolved zirconium content of the bath must be determined. One method of determining the dissolved zirconium content of an etching bath is to remove a sample of the bath and analyze the same in a laboratory to ascertain the zirconium content, a time consuming and costly process. Other faster and less costly procedures have been proposed, but as described in published German Patent Disclosure No. 28 28 547, the determination of zirconium metal content in hydrofluoric acid containing etching baths by colorimetric or titrimetric methods is not feasible. In said German patent disclosure, the zirconium content of a hydrofluoric acid-nitric acid etching bath is determined by drawing off a portion of the bath, precipitating the metal in the portion in a form of a difficult to dissolve compound, and determining the concentration of the difficult to dissolve compound in a diluting agent by measuring the turbidity thereof. The preferred precipitating agent is a solution of caustic soda which precipitates the zirconium in the form of zirconium hydroxide, and water to adjust to the necessary dilution.
It is an object of the present invention to provide an efficient and economical process for determining the zirconium metal content of a hydrofluoric acid-nitric acid zirconium metal etching bath.